1. Field of the Invention
This invention relates to a process using a high activity catalyst for producing cross-linkable polymers of ethylene with 1,4-hexadiene or of ethylene with 1,4-hexadiene and a 1-olefin which have high contents of functionalizeable pendant groups and which have a narrow molecular weight and comonomer distribution.
2. Background
It is highly desirable to tailor polymers and copolymers of 1-olefins to produce products having specific desirable properties. It is known that the polymerization of ethylene with 1,4-hexadiene or the polymerization of ethylene with 1,4 hexadiene and a 1-olefin will produce a polymer having 2-butenyl free branched chains. Such a polymer has special value because it is cross-linkable through these branched chains to form a high strength crystalline composition. In addition to this cross-linkable feature, such a polymer can also be easily functionalized through the chemical reaction of the 2-butenyl branching with other chemicals to form a functionalized polyolefin.
For many end use applications polyethylene (PE) must be first treated to incorporate into the carbon-carbon backbone pendant groups containing reactive functional groups. For instance, to prepare PE to receive and retain printing, etc., the PE must be functionalized, as for example by treating it in a peroxide catalyzed reaction with maleic anhydride. A disadvantage of functionalizing PE by such a maleation reaction is that the peroxide catalyst also cross-links PE, thereby causing gel in films and the like.
As a possible alternative to the functionalization of PE by a peroxide catalyzed maleation reaction, some efforts have been directed to producing a PE copolymer having pendant groups which can be functionalized without cross-linking. Such efforts have included copolymerizing ethylene with a diolefin such as 1,4-hexadiene. Copolymerization of ethylene with 1,4-hexadiene yields a polyethylene-type carbon backbone wherein 2-butenyl groups are pendant from the carbon-carbon backbone at each site wherein a 1,4-hexadiene comonomer has been incorporated. The double bond of the 2-butenyl pendant group may be reacted with common reagents under mild conditions which will not cross-link the PE resin and thereby be readily converted to a variety of functional groups, thus providing the desired functionalized PE.
The copolymerization of ethylene with .alpha.-olefins, such as propylene, 1-butene, 1-pentene, 1-hexene, is well known and practiced for the purpose of producing a variety of PE resin types, such as linear low density polyethylene (LLDPE). The copolymerization of ethylene with .alpha.-olefins proceeds readily in the presence of Ziegler-Natta type catalysts and also the more recently developed catalyst comprising a metallocene and alumoxane. Ziegler-Natta type catalysts generally comprise a Group IVB metal compound such as Ti halide and an aluminum alkyl cocatalyst. The metallocene-containing type catalyst is one of a Group IVB metal metallocene (i.e., a coordination compound of a Group IVB metal with cyclopentadienyl ligands) cocatalyzed with alumoxane. A metallocene, especially a zirconocene, is generally not active as a catalyst when cocatalyzed with an aluminum alkyl as in the traditional Ziegler-Natta type catalyst, but a zirconocene generally has a vastly higher catalytic activity than traditional Ziegler-Natta type catalyst when cocatalyzed with an alumoxane. An advantage of the zirconocene/alumoxane catalyst system is that it catalyzes the production of ethylene and .alpha.-olefin homopolymers and copolymers to a narrower molecular weight distribution than that obtainable with traditional Ziegler-Natta type catalysts.
Attempts to utilize a Ziegler-Natta or a metallocene/alumoxane type of catalyst to copolymerize ethylene with a diolefin have heretofore been less than satisfactory for commercial purposes because of the low polymerization activities of such catalyst when subject to the presence of a diolefin monomer.
Japanese Kokai numbers 119,215 (published May 30, 1987); 121,707(published June 3, 1987); and 121,709 (published June 3, 1987) disclose production of soft copolymers variously of ethylene-.alpha.-olefin, propylene-.alpha.-olefin, butylene-.alpha.-olefin, using a metallocene/alumoxane catalyst system wherein the metallocene is a metal salt of a lower alkylene bridged -bis(cyclopentadienyl), -bis(indenyl) or -bis(tetrahydroindenyl) compound. The Japanese Kokai represent that copolymer products may be produced by a gas or liquid phase reaction procedure to have a wide range of properties such as crystallinities from 0.5-60%, while having a molecular weight distribution (MWD) less than 3 with low levels of boiling methyl acetate soluble components.
European Patent Application 206,794 discloses that certain supported metallocene/alumoxane systems, particularly bis(cyclopentadienyl) transition metal metallocenes are useful for polymerizing ethylene to a homopolymer or to a copolymer with an olefin or diolefin for purposes of modifying the clarity or impact properties of the polyethylene polymer product. The patent is directed to a supported metallocene catalyst which may be used in the production of copolymers of ethylene in slurry or gas-phase processes in the presence of small quantities of alumoxane cocatalyst.
Efforts to utilize zirconocene/alumoxane catalyst for the production of olefinic elastomers, such as ethylene-propylene-diene (EPDM), have been reported. To date, however, these reports have not been encouraging to a belief that a zirconocene/alumoxane catalyst system would have a sufficiently high catalyst activity when subject to the presence of a 1,4-hexadiene monomer for commercial utilization in the production of an ethylene with 1,4-hexadiene copolymer. Kaminsky, J. Poly. Sci., Vol. 23, pp. 2151-64 (1985) reports upon the use of a soluble bis(cyclopentadienyl) zirconium dimethyl/alumoxane catalyst system for toluene solution polymerization of elastomers containing ethylene, propylene and 5-ethylidene-2-norbornene (ENB). Kaminsky employed this catalyst at low zirconium concentrations, high Al:Zr ratios and long reaction times to prepare, in low yields, high molecular weight EPDM elastomers having high ENB incorporation.
Similar to Kaminsky, Japanese Kokai 121,711 (published June 3 , 1987), illustrates the use of a soluble bis(cyclopentadienyl) zirconium monohydride monochloride/alumoxane catalyst system for toluene solution polymerization of ethylene and butene-1 wherein, variously 5-ethylidene-2-norbornene (ENB), 5-vinylidene-2-norbornene (VNB), and dicyclopentadiene (DCPD) were employed as the diene. Japanese Kokai 121,711 further suggests, but does not illustrate, that the zirconocene component of the catalyst system may be a bis(indenyl) zirconium hydride or bis(tetrahydroindenyl) zirconium hydride rather than a bis(cyclopentadienyl) zirconium hydride. Although Japanese Kokai 121,711 suggests that .alpha.-olefins other than 1-butene can be employed, it illustrates only the production of an ethylene-butene-1-diene elastomer (EBDM) material in a continuous flow atmospheric pressure reaction.
It is desirable to develop a process for the copolymerization of ethylene with 1,4-hexadiene and the polymerization of ethylene with 1,4-hexadiene and a 1-olefin which would proceed at commercially useful rates of polymer production in the presence of sufficiently small quantities of catalyst such that the product polymer would not require deashing in a subsequent process to remove catalyst residue from the polymer product.
It would be still further desirable that the process provides for a relatively high level of 1,4-hexadiene comonomer insertion, on the order of from about 0.1 to about 10.0 mole % of the polymer product, at 1,4-hexadiene monomer concentrations which do not depress the activity of the catalyst utilized in the process below levels which are in the range of commercial utility. It would be even more desirable that the process be one which is capable of producing the polymer directly in particle form, such as in a gas phase polymerization process wherein the catalyst employed is in solid or supported particulate form.